Plasma is an electrically neutral medium of unbound positive and negative particles. It is important to note that although the particles are unbound, they are not ‘free’ in the sense of not experiencing forces. When a charged particle moves, it generates an electric current with magnetic fields; in plasma, the movement of a charged particle affects and is affected by the general field created by the movement of other charges. This governs collective behavior with many degrees of variation. For plasma to exist, ionization is necessary. The term “plasma density” by itself usually refers to the “electron density”, that is, the number of free electrons per unit volume. The degree of ionization of a plasma is the proportion of atoms that have lost or gained electrons, and is controlled mostly by the temperature. Even a partially ionized gas in which as little as one percent of the particles are ionized can have the characteristics of a plasma, such as response to magnetic fields and high electrical conductivity.